1. Field of the Invention
This invention relates to printing systems and methods and particularly systems and methods using multiple scan beams that have wide lateral separations.
2. Description of Related Art
Printing systems including scanners are suitable for a variety of applications including printing text on paper, patterning photoresist during integrated circuit manufacture, and creating masks or reticles for projection-type photolithography systems. For integrated circuit applications, the printing systems typically require submicron precision. FIG. 1A illustrates the basic architecture of a precision printing systems 100 that employs scanning. System 100 includes: a light source 110 such as a laser; an acousto-optic modulator 120 that controls intensity of one or more input beams 135; pre-scan optics 130 that control the position, shape, and collimation of input beams 135; a scanning element 140 such as a polygon mirror that sweeps scan beams 145 along a scan direction; and post-scan optics 150 that focus scan beams 145 on an image plane 160. Scanning of scan beams 145 forms scan lines that expose a pattern in an image area of plane 160. Acousto-optic modulator 120 modulates the intensity of input beams 135 to select the pattern that scan beams 145 expose.
A conventional acousto-optic modulator includes a block of material such as fused silica through which input beams propagate. To turn on, turn off, or change the intensity of an input beam, a transducer generates an acoustic wave that crosses the path of the input beam in the block. The acoustic wave locally changes the optical properties of the block and deflects part of the input beam. Typically, a beam stop later in the optical train blocks the undeflected portion of the beam.
A concern for a precision scanner having a conventional acousto-optic modulator is the orientation of the scanning direction relative to propagation of the acoustic waves that modulate the input beams. If the propagation direction and the scanning direction are not collinear, the turning on and turning off of beams can reduce sharpness of edges or create undesired skew or directional bias in a pattern being illuminated. FIG. 1B illustrates an illuminated region 170 of a scan line formed when an acoustic wave deflects an input beam in a direction 178 that (after convolution through the system optics 130 and 150) is perpendicular to a scan direction 172. Deflection direction 178 typically corresponds to the direction of propagation of the acoustic wave in the acousto-optic modulator. As acousto-optic modulator 120 turns on input beam 135, a cross-section 174 of the beam expands in direction 178. Accordingly, the initially illuminated part of region 170 is narrow and toward one edge until the input beam has a fully illuminated cross-section such as cross-section 175. Similarly, when acousto-optic modulator 120 turns off input beam 135, one edge of the input beam darkens first, and a shrinking cross-section 176 of the beam causes illuminated region 170 to recede toward the opposite edge.
This reduces sharpness at the edges of illuminated regions formed by multiple scan lines, skews rectangular illuminated areas, and causes pattern lines at 45° to the scan direction to differ in thickness from pattern lines at 135° to the scan direction.
However, to provide independent control of the beam intensities and a narrow scan brush, acoustic waves in an acousto-optic modulator generally propagate at an angle relative to the scan direction.
As shown in FIG. 1C, a separation 133 between beams 132, 134, 136, and 138 inside acousto-optic modulator 120 must be sufficient for acoustic waves 122, 124, 126, and 128 to independently modulate respective beams 132, 134, 136, and 138. Typically, separation 133 must be more than a beam diameter. To avoid the separation causing gaps between scan lines, a scanning direction 172 is selected so that beams 132, 134, 136, and 138 overlap when viewed along the scan direction 172. An advantage of overlapping beams is the narrow width 180 of the scan brush. Narrow brushes reduce scan line bow which is common for conventional f-θ scan lenses. (Scan line bow is the curvature of scan lines that are off the optical axis of a scan lens.) Also, scanning overlapping beams along scan direction 172 forms a band of scan lines without intervening gaps, which simplifies indexing of scan lines to cover the image area. As indicated above, disadvantages of the configuration of FIG. 1C are reduced sharpness at the edges in the image, skew of rectangular areas, and 45°/135° line thickness bias.
As shown in FIG. 1D, the scan direction 172 can alternatively be the same as or opposite to the direction of propagation of acoustic waves 122, 124, 126, and 128 in acousto-optic modulator 120. With this configuration, the separation 133 required for independent modulation of beams controls the separation between the scan lines. This creates a scan brush that is wider than the brush of FIG. 1C, and the wider scan brush increases scan line bow from a conventional f-θ scan lens, making the accuracy required for integrated circuit applications difficult to achieve. Other types of scan lenses can reduce scan line bow but generally cause scan beams to move with non-uniform velocity and therefore can distort the image.
Systems and methods are sought that use simultaneous scan beams for faster scanning but avoid scan line bow and image distortion and also avoid the skew, blurred edges, and directional bias associated with acousto-optic modulators having acoustic waves propagating at an angle to the scan direction.